The present invention relates to a method of drying a thermoplastic norbornene resin used as a raw material for the substrate of a magnetic recording medium mounted on the external memory of a computer and such a magnetic memory for storing digital data. The present invention also relates to a substrate for magnetic recording media containing the thermoplastic norbornene resin dried by the method according to the invention, a magnetic recording medium including the substrate according to the invention and a method of manufacturing the magnetic recording medium, such as a hard disk, using the substrate according to the invention.
As the capacities of the storage apparatuses that use magnetic recording media become larger, the flying heights of the magnetic heads have been lowered to improve the recording densities. The magnetic recording medium must have a very flat and smooth surface, that is to have a very precise surface structure, to lower the flying height of a magnetic head. For example, it is required that the conventional nonmagnetic metal substrate, such as an aluminum (Al) substrate, be machined precisely.
The conventional nonmagnetic metal substrate and the conventional magnetic recording medium using the conventional nonmagnetic metal substrate are manufactured in the following way.
Usually, a blank disk, prepared by rolling a molten metal, heating the rolled metal, annealing the heated metal and machining the annealed metal to have the predetermined dimensions, is used for a nonmagnetic substrate. The blank disk is machined to have a predetermined inner diameter and a predetermined outer diameter. The flatness and the smoothness of the machined blank disk is improved by lapping. A Nixe2x80x94P layer of 13 xcexcm in thickness is plated on the blank disk to improve the surface hardness thereof. The surface of the Nixe2x80x94P layer is polished to a surface roughness Ra of 10 xc3x85. Final lapping using diamond slurry is applied to the polished surface of the blank disk. Laser zone textures are formed in the contact start stop (CSS) zone of the thus obtained substrate such that the bump height is, for example, 190 xc3x85 and the bump density is, for example, 30xc3x9730. Finally, the substrate is washed meticulously, resulting in a substrate for magnetic recording media.
A chromium (Cr) undercoating layer of 500 xc3x85 in thickness, a Co-14Cr-4Ta magnetic layer of 300 xc3x85 in thickness and a carbon protection layer of 80 xc3x85 in thickness are deposited one after another on the substrate for magnetic recording media by the DC sputtering method. The surface of the as deposited laminate is burnished using a burnishing tape and a fluorine lubricant layer of 20 xc3x85 in thickness is formed on the burnished surface by dip-coating or by spin-coating, resulting in a magnetic recording medium.
The method of manufacturing the conventional substrate for magnetic recording media and the method of manufacturing the conventional magnetic recording medium are becoming more complicated to meet the recent requirements for a higher recording density. Moreover, it is required to manufacture a magnetic recording medium with reduced costs while maintaining high functions. Novel magnetic recording media, that use a plastic substrate, have been proposed to meet these contradictory requirements.
A method that manufactures a plastic substrate by molding and, at the same time, forms the CSS zone thereof with excellent productivity, is advantageous to industrially provide magnetic recording media with low manufacturing costs.
However, the plastic substrates manufactured by injection molding synthetic resin pellets are inferior to the metal substrates and the ceramic substrates, such as a glass substrate, from the view points of surface flatness and smoothness, since rugged surface portions of several xcexcm in height difference and micro waviness are caused by molding. The rugged portions and the micro waviness are hazardous for realizing a very flat and smooth surface, which is a prerequisite for the magnetic recording medium.
The magnetic recording medium formed on the substrate including the rugged portions and the micro waviness is also hazardous for reading out data to the magnetic head and for writing data from the magnetic head. Especially when a low-flying-height head conducts continuous seek at a high speed, the flight thereof is not stabilized and, in the end, head crash is the result. Thus, the conventional plastic substrate impairs the durability of the magnetic recording medium.
Polycarbonate resins and poly(methyl methacrylate) resins are used for the material of the plastic substrate for magnetic recording media. In addition to these resins, thermoplastic norbornene resins are used for the material of the plastic substrate for magnetic recording media. The thermoplastic norbornene resin, that exhibits excellent thermal resistance, low hygroscopicity and excellent shape stability, is useful to provide excellent magnetic recording media. However, the problems described above are posed also on the substrate made of the thermoplastic norbornene resin.
The commercial products of the thermoplastic norbornene resin include APEL supplied from Mitsui Chemicals, Inc. and ZEONEX supplied from Nippon Zeon Co., Ltd.
In view of the foregoing, it is a first object of the invention to provide a method of drying a thermoplastic norbornene resin for controlling the specific gas components therein below certain levels.
It is a second object of the invention to provide a very reliable plastic substrate, that contains less rugged portions and low micro waviness due to the use of a thermoplastic norbornene resin wherein the specific gas components therein are suppressed below certain levels by the method of drying described above.
It is a third object of the invention to provide a magnetic recording medium using the plastic substrate described above. More specifically, it is the third object of the invention to provide a magnetic recording medium that has a very flat and smooth surface with less rugged portions and low micro waviness and exhibits excellent durability against continuous and high-speed head seek.
It is a fourth object of the invention to provide a method of manufacturing the magnetic recording medium described above.
The extensive and intensive studies conducted by the present inventors have revealed that the foregoing objects of the invention are achieved by adjusting the gas components contained in a thermoplastic norbornene resin below certain levels by a specific drying method, and by manufacturing a magnetic recording medium using the dried resin.
According to a first aspect of the invention, there is provided a method of drying a thermoplastic norbornene resin, the method including drying the thermoplastic norbornene resin under vacuum or under the ordinary pressure and under vacuum to remove atmospheric gas components including N2, O2 and H2O and low-boiling-point organic components (organic impurities) including aliphatic components and aromatic components contained in the resin. The aliphatic components include low-molecular-weight aliphatic hydrocarbons and oxidation products of the resin such as alcohol compounds and carboxylic acid compounds. The aromatic components include the residues of the solvents used for synthesizing the resin.
Advantageously, the drying under the ordinary pressure is conducted at any temperature between 80 and 120xc2x0 C., and the drying under vacuum is conducted under the degree of vacuum of 20 Pa or lower and at any temperature between 80 and 120xc2x0 C.
Advantageously, the thermoplastic norbornene resin contains, after the drying, N2 of 20 ppm or lower, O2 of 20 ppm or lower, H2O of 1 ppm or lower, aliphatic components, that are low-boiling-point organic components, of 20 ppb or lower in total, and aromatic components, that are the other low-boiling-point organic components, of 20 ppb or lower in total.
According to a second aspect of the invention, there is provided a plastic substrate for magnetic recording media manufactured by injection molding the thermoplastic norbornene resin dried by any of the methods described above.
Advantageously, the substrate contains in the surface thereof 100 or less rugged portions of 1 xcexcmxc3x971 xcexcm or wider in area.
Advantageously, the straightness Pa in the radial direction of the substrate is 1 xcexcm or less, the micro waviness of the substrate is 500 xc3x85 or lower, and the average surface roughness of the substrate is 5 xc3x85 or lower.
According to a third aspect of the invention, there is provided a magnetic recording medium including a substrate, a magnetic layer above the plastic substrate, a protection layer on the magnetic layer, and a lubricant layer on the protection layer, the substrate being the plastic substrate described above.
Advantageously, the output of a strain gauge is 0.5 g or less at the end of continuous and high-speed head seek tests conducted for 24 hr on the magnetic recording medium rotating at 4500 rpm using a low-flying-height head, the flying height thereof is 1 xcexcxe2x80x3.
According to a fourth aspect of the invention, there is provided a method of manufacturing a magnetic recording medium, the method including the steps of drying a thermoplastic norbornene resin by the method described above, injection-molding the dried thermoplastic norbornene resin to form a plastic substrate, forming a magnetic layer above the plastic substrate, forming a protection layer on the magnetic layer, and forming a lubricant layer on the protection layer.